This invention relates to a delivery system for conveying and conditioning molten thermoplastic material, such as molten glass, from a source of such material, such as a melting furnace, to a desired outlet position, such as adjacent forming apparatus. In its preferred form, the invention pertains to a forehearth for the delivery of molten glass from a furnace wherein the molten glass is conveyed through a refractory metal conduit and is simultaneously conditioned to a desired relatively uniform temperature and viscosity by means of a heat exchange device for efficiently and economically providing heat to and removing heat from the molten glass as required, as it passes therethrough. The conduit may be immersed in, or surrounded by, stagnant glass. Water cooled inductive heating means is located about the conduit in intimate contact with refractory material positioned about the conduit to provide uniform heat input into the glass within the conduit while maintaining a relatively cool exterior refractory surface.
Although by no means limited to such use, the present invention has particular applicability for use with vertically oriented electric glass melting furnaces such as disclosed in U.S. Pat. Nos. 4,029,887, and 4,143,232. Similar delivery systems have in fact been utilized with such vertical melting furnaces as shown in U.S. Pat. Nos. 4,352,687, 4,365,986 and 4,365,987. U.S. Pat. No. 4,365,987 is similar to U.S. Pat. No. 4,352,687 patent but with the inclusion of a flow control means for regulating flow through the delivery conduit.
The glass delivery system disclosed in U.S. Pat. No. 4,352,687 includes a refractory metal pipe or conduit enclosed within a refractory structure for conveying molten glass. The refractory structure is spaced about the pipe and defines a closed space which receives a quantity of the glass so as to surround the outer periphery of the pipe with such glass. Heat input devices, in the form of molybdenum mesh electric heaters, are positioned within the closed space for supplying heat to the pipe. In addition separate cooling means are provided for removing heat energy given up by the glass delivered through the pipe.
The delivery systems disclosed in Pat. Nos. 4,365,986 and 4,352,687 have not been completely satisfactory, since the molybdenum mesh heaters are subject to corrosion and oxidation failure. Without the availability of the heat supplied by such molybdenum mesh heaters, the delivery systems set forth in such patents cannot be relied upon to maintain desired glass temperatures in normal operations, or an ability for the glass to soak at temperature, or the ability to be cooled down and brought back up to an operating temperature.
Unlike the delivery systems disclosed in U.S. Pat. Nos. 4,365,986 and 4,352,687, the water cooled inductive system of the present invention does not require power feed throughs or power feed through thermocouples, or the associated purge lines. The refractory construction of the present invention is less voluminous, since the pipe-to- refractory cavity can be several inches tighter, and the refractory thickness can be minimized since the water-cooled inductive coils provide a positive barrier to glass leakage by setting up a large temperature gradient across the refractory. In addition, the cooling rate obtainable with U.S. Pat. No. 4,352,687 the delivery system can be increased by the present system since the refractory sections are thinner and the water cooled induction coils are in intimate contact with the refractory. It is also felt that the induction heated system is more reliable than the mesh heater powered system of U.S. Pat. No. 4,352,687.